1. Field of the Invention
The present invention relates to a fiber composite material having improved orientation of fibers and excellent friction resistance with respect to ice, and to a pneumatic tire in whose tread portion the fiber composite material is used and which has markedly improved running performances, such as braking, traction and cornering, on icy roads. (Such performances will be referred to hereinafter as "on-ice performances".)
2. Description of the Related Art
Characteristics of a matrix material are often improved by using a material having elasticity, such as a rubber or an elastomer, as a matrix and compounding fillers, fibers or the like into the matrix. For example, Japanese Patent Application Laid-Open (JP-A) No. 4-38206 discloses a method of controlling the rigidity of a tire by adding short fibers to a matrix rubber. Japanese Patent Application Laid-Open (JP-A) No. 3-152140 discloses a method of improving the on-ice characteristics due to the dropping off of fibers which have been added into a matrix rubber of a tire. In this way, by compounding short fibers into the matrix such that the short fibers either adhere or do not adhere to the matrix, a material having various functions can be obtained.
The functions realized by the compounding of the short fibers are brought about by the orientation of the short fibers within the matrix.
Generally, short fibers 10 illustrated in FIG. 1 are subjected to stress, heat and the like at the time of being compounded with a matrix 12 so as to become short fibers 14 (see FIG. 2) which have shrunk due to heat shrinkage. In this shrunken state illustrated in FIG. 2, the short fibers 14 are compounded with the matrix 12. Accordingly, because the composite material contains shrunken fibers, the functions which are obtained by the orientation of the fibers deteriorate.
Because the orientation of the short fibers in a conventional short fiber composite material deteriorates, it is difficult to control the physical properties resulting from the compounding of short fibers, anisotropy of the short fibers worsens, and functions such as the on-ice performance of a pneumatic tire in whose tread portion such a material is used deteriorate. This phenomenon is more marked in cases in which the short fibers have not undergone adhesion processing for adhering the short fibers to the matrix than in cases in which the short fibers have been subjected to such adhesion processing.
Japanese Patent Application Laid-Open (JP-A) Nos. 4-38206, 4-176707 and 4-176708 for example disclose, as a pneumatic tire provided with a foamed rubber layer (hereinafter "foamed tire"), a tire which is used as a studless tire and in which foamed rubber and short fibers are used in the tire tread. The technologies disclosed in these publications for improving on-ice performance orient the fibers in the tire circumferential direction and increase the rigidity in order to improve the edge effect. On the other hand, in these technologies, attempts are made to not raise the rigidity by disposing the fibers to run along the direction orthogonal to the tire circumferential direction, because lower rigidity is important for the adhesive friction effect (hereinafter, "adhesive effect") which occurs when the rubber contacts an icy road surface. In this way, both the edge effect and the adhesive effect can be established simultaneously.
However, in actuality, even if the fibers are oriented and compounded as described above, among fibers which have been subjected to adhesion processing to adhere to the rubber as well as fibers which have not been subjected to such processing, those fibers which are smaller than a certain size result in an increase in the rigidity of the tread rubber also in a direction orthogonal to the direction of orientation due to the filler effect. Further, when foaming is also carried out, the orientation of the fibers in the circumferential direction of the tire becomes disordered, and in an actual studless tire in which grooves known as sipes are formed in a dense concentration in the tread portion, when the tire is vulcanized, the orientation of the compounded fibers becomes even more disordered due to blades which are used for forming the sipes. As a result, the tire radial direction rigidity, which is important to the adhesive effect, increases, and hardly any improvement in the on-ice performance can be observed.
Japanese Patent Application Laid-Open (JP-A) No. 63-89547 discloses a foamed rubber composition in which a fibrous material is randomly compounded. In this case, although orientation of the fibers is not carried out, the modulus of elasticity increases in all directions so that there is little improvement in the on-ice performance.
Japanese Patent Application Laid-Open (JP-A) No. 4-38207 discloses improving the water-removing effect and the on-ice performance by mixing short fibers having poor adhesion with foamed rubber so that the foam gas gathers around the short fibers and forms continuous cells. However, in actuality, the foaming reaction is not concentrated around the short fibers. However, because short fibers having poor adhesion to rubber are compounded, it is easy for the fibers to drop off due to actual use of the tire on a traveling vehicle. Such dropping off leaves concave portions in the tire, which result in a water-removing effect. However, when rigid fibers such as aramide fibers or carbon fibers are used, the hardness of the entire rubber increases, which adversely affects the adhesive effect. On the other hand, as described above, when general organic fibers such as nylon fibers or polyester fibers are used, the fibers heat-shrink during the molding process, and the configuration of the fibers changes from the configuration illustrated in FIG. 1 to that of FIG. 2. Therefore, the flow of water to be removed from the ground-contact surface of the tire is not smooth, and it is difficult for water to be effectively removed from the ground-contact surface, so that a sufficient improvement in the on-ice performance cannot be obtained. Further, when such organic fibers are used, in this case as well, the drawback arises in that if the fibers are smaller than a given size, a filler effect occurs and the hardness of the rubber increases.
As has been described above, conventional combinations of foamed rubber and short fibers have resulted in drawbacks such as an increase in the hardness of the rubber. Although the running performances on dry roads and wet roads may improve, the improvement in on-ice performance, which is a fundamental object of studless tires, has been insufficient.